As a technique for imparting water repellency and oil repellency at the same time to a surface of an article, it has been common to treat the article with an organic solvent solution or an aqueous dispersion of a polymer containing polymerized units of a polymerizable monomer containing a polyfluoroalkyl group (a group having a structure wherein at least two and at most all of hydrogen atoms in an alkyl group are substituted by fluorine atoms, such a polyfluoroalkyl group will hereinafter be referred to as an “Rf group”) in its molecule or a copolymer of such a polymerizable monomer with other monomers.
The development of such water and oil repellency is attributable to formation of “a surface having a low surface energy” where the critical surface tension is low, which is formed on the surface of the coating film by a surface orientation of Rf groups. It has been considered that in order to develop both water repellency and oil repellency, the orientation of Rf groups on the surface is important, and in order to realize the surface orientation of Rf groups, it is necessary for the polymer to have structural units based on a monomer having a perfluoroalkyl group (a group having a structure wherein all hydrogen atoms in an alkyl group are substituted by fluorine atoms, such a perfluoroalkyl group will hereinafter be referred to as an “RF group”) having at least 8 carbon atoms.
However, recently, EPA (United States Environmental Protection Agency) has pointed out that a compound having an RF group having at least 8 carbon atoms is decomposed in an environment or a living body and its decomposition product is accumulated there, thus bringing about a high environmental burden. Accordingly, a copolymer for a water/oil repellent composition having structural units based on a monomer having an RF group having at most 6 carbon atoms and containing no structural units based on a monomer having an RF group having at least 8 carbon atoms, is required.
As such water/oil repellent compositions, a water/oil repellent composition containing, as an effective component, a copolymer comprising polymerized units based on a monomer having an Rf group (preferably an RF group having at most 6 carbon atoms) with a low melting point of fine crystals and polymerized units based on a monomer having no Rf group with a high melting point of fine crystals (Patent Document 1), and a water/oil repellent composition containing, as an effective component, a compound comprising polymerized units based on a monomer having an Rf group (preferably an RF group having at most 6 carbon atoms) having a glass transition point of at least a certain temperature with a low melting point of fine crystals and polymerized units based on a monomer having a crosslinkable functional group without having an Rf group (Patent Document 2) are known.
However, each of these water/oil repellent compositions is one for the purposes of (1) imparting water and oil repellency to an article by treatment even at a low temperature, (2) enabling water and oil repellent processing to provide soft texture and (3) enabling water and oil repellent processing to provide excellent durability, and has a problem such that the oil repellency is not sufficient although the water repellency and its durability are excellent.
Further, with respect to a copolymer having structural units based on a monomer having an Rf group having at most 6 carbon atoms, one capable of exhibiting sufficient oil repellency against oils having low surface tensions, such as gasoline or a hydrocarbon liquid with a low surface tension, has not been known.